In general, for a construction machine, for example, a small excavator, a hydraulic control system is adopted which is configured to enable traveling and to actuate a working device and an option device by controlling flow rates and flows of pressurized oils that are to be discharged from a plurality of hydraulic pumps through a main control valve (MCV) having a plurality of spools. For example, one hydraulic pump is configured to supply the pressurized oil to a left spool for traveling, a spool for a boom, a spool for a bucket and a spool for arm confluence. Also, the other hydraulic pump is configured to supply the pressurized oil to a right spool for traveling, a spool for an arm, a spool for an option device and a spool for boom confluence. In the case of the hydraulic control system in accordance with the related art, a spool for confluence may not be provided. However, in order to increase operating speeds of a boom and an arm with limited flow rates of the hydraulic pumps, a confluence system of the pressurized oils to be discharged from the respective hydraulic pumps is applied in most cases.
However, in the related art, even when the confluence system of the pressurized oils is applied, a desired operating speed is not obtained, so that a higher operating speed is required. Regarding this, if a capacity of the hydraulic pump is increased, the prime cost increases and an exterior size of the hydraulic pump increases, which makes it difficult to mount the hydraulic pump to the small excavator. Also, in the related art, even a bucket and an option device for which the high operating speed is not required are operated at high operating speeds upon the joining of the pressurized oils, so that there is a limitation in increasing the capacity of the hydraulic pump with no particular plan.
In the meantime, in a case where swing and a dozer are not used, an internal structure of the MCV is changed to enable a flow rate of the pressurized oil, which is discharged from a third hydraulic pump configured to supply the pressurized oil to the MCV, to join pressurized oil, which is supplied to a hydraulic actuator configured to actuate the boom or arm, thereby increasing the operating speed of the boom or arm. According to this method, however, it is necessary to considerably change the internal structure of the MCV.